It's like trading shares.
You buy some bitcoins from the exchange, and generate more using them as seed.
However, it has become so difficult now, that competition is getting very fierce, there are enthusiasts for this thing as well.
Earlier the people having the hardware were making money, now the people in the exchange are.
The thing is that, the price of one bitcoin is very high, that's making this a very attractive proposal(Somewhere around $125).
The litecoin is easier to generate than the bitcoin, so people are mining them instead now.